Device for controlling lighting time of lamp

ABSTRACT

A series circuit consiting of a device for obtaining a constant voltage and a resistor, a series circuit consisting of a resistor and a charging-discharging circuit adapted to be charged to a predetermined level when a trigger switch is closed and then gradually discharged, and a circuit consisting of a lamp, a switching circuit and its output circuit, are connected in parallel to each other and to a DC power source. The control voltage and the reference voltage of the switching circuit are derived from the junction between the charging-discharging circuit and the resistor connected in series thereto and the junction between the device for obtaining a constant voltage and the resistor connected in series thereto, respectively. When the voltage of the DC power source drops below the rated voltage, the voltage difference between said two junctions is increased so that the &#39;&#39;&#39;&#39;on&#39;&#39;&#39;&#39; time of the switching circuit is increased, thereby increasing the lighting time of the lamp.

United States Patent T akahashi 1 1 DEVICE FOR CONTROLLING LIGHTING TIME OF LAMP [75] Inventor: Akira Takahashi, Tokyo, Japan [73] Assignee: Ricoh Co., Ltd., Tokyo, Japan [22] Filed: Nov. 2, 1973 [21] App1.No.:412,097

[56] References Cited UNITED STATES PATENTS 2,923,863 2/1960 Chcsson et a1 307/293 X 3,192,449 6/1965 Brockctt 307/293 X OTHER PUBLlCATlONS Demjancnko, Push-Button Single-Pulse Generator is More Dependable, Electronic Design, Aug., 3, 1964, p. 43.

[4 1 Sept. 16, 1975 Primary Examiner-James B. Mullins Attorney, Agent, or Firm-Cooper, Dunham, Clark, Griffin & Moran ABSTRACT A series circuit consiting of a device for obtaining a constant voltage and a resistor, a series circuit consisting of a resistor and a charging-discharging circuit adapted to be charged to a predetermined level when a trigger switch is closed and then gradually discharged, and a circuit consisting of a lamp, a switching circuit and its output circuit, are connected in parallel to each other and to a DC power source. The control voltage and the reference voltage of the switching circuit are derived from the junction between the charging-discharging circuit and the resistor connected in series thereto and the junction between the device for obtaining a constant voltage and the resistor connected in series thereto, respectively. When the voltage of the DC power source drops below the rated voltage, the voltage difference between said two junctions is increased so that the on time of the switching circuit is increased, thereby increasing the lighting time of the lamp.

5 Claims, 3 Drawing Figures PATENTEB SEP I 8 I875 FIG.

TIME

FIG.

DEVICE FOR CONTROLLING LIGHTING TIME OF LAMP BACKGROUND OF THE INVENTION The present invention relates to a device for controlling a lighting time of a lamp.

An illumination device incorporating the so-called CR discharge type timer consisting of a capacitor and a resistor connected in parallel thereto is used in a camera of the type capable of superimposing desired data such as dates upon a film together with a subject. A trigger switch is actuated in response to the shutter release operation or by a suitable actuating member to energize the illumination device to turn on a lamp, thereby illuminating desired data to be superimposed upon a film. However, when the voltage of the power source for the illumination device drops, the quantity of light emitted from the lamp is reduced and the lighting or on time of the lamp, which is determined by the CR timer, is also decreased, resulting in the underexposure of the data to be superimposed. It is economically disadvantageous to replace a power source such as a dry cell too frequently, but when the power source whose voltage drops below a certain level is not replaced, the failure in data superimposition occurs.

SUMMARY OF THE INVENTION In view of the above, the present invention provides an improved device for controlling a lighting time of a lamp, in which a device for obtaining a constant voltage is inserted into a CR timer so that the on time interval of the timer may be increased as the voltage drop of the power source is increased, thereby preventing the drop in quantity of light emitted from the lamp.

Briefly stated, according to the present invention, a charging-discharging circuit adapted to be charged to a predetermined constant voltage when a trigger switch is closed and to start the discharge as soon as the trigger switch is opened, a switching circuit for controlling the current flowing through a lamp, and a series circuit consisting of a resistor and a device such as a zener diode for obtaining a constant voltage, are connected to parallel to each other and to a DC power source. The base of a switching element in the first stage of the switching circuit is connected to the chargingdischarging terminal to be referred to as the point a hereinafter of the charging-discharging circuit, and the emitter is connected to the junction of the device such as a zener diode for obtaining a constant voltage and the resistor in said series circuit. (the latter junction will be referred to as the point b hereinafter.) The switching element is turned on when the voltage difference between the points a and b is within a predetermined range, so that a lamp is turned on. The voltage at the point b varies as the voltage of the DC power source varies. The charging-discharging circuit is always charged to a predetermined constant voltage so that the larger the voltage drop of the DC power source. the longer the time when the voltage at the points a and b are equal becomes. As a result, the lighting time of the lamp is increased accordingly.

The present invention will become more apparent from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a circuit diagram of a first embodiment of the present invention;

FIG. 2 is a graph used for the explanation of the mode of operation thereof; and

FIG. 3 is a circuit diagram of a second embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS:

First Embodiment, FIGS. 1 and 2 FIG. 1 is a diagram of a circuit for turning on a lamp L in order to superimpose the data such as date upon a film loaded in a camera. SW is a trigger switch which is closed for a very short time and then opened in response to a shutter release operation or by a suitable actuating member. R, is a resistor; R a variable resistor; R a resistor; C, a capacitor connected in parallel with the variable resistor R D,, a device such as a zener diode for obtaining a predetermined constant voltage connected in parallel with the variable resistor R and the capacitor C; Tr, and Tr NPN transistors; and Tr a PNP transistor. These transistors Tr,, Tr and Tr,, make up a switching and amplifier circuit. E is a power source; and D a device, such as a zener diode for obtaining a predetermined constant voltage, connected in parallel with the lamp L and the transistor Tr and connected in series to the resistor R and the power source E.

Next the mode of operation will be described. When the trigger switch SW is closed for a very short time, the capacitor C is charged. The voltage across the capacitor C has a predetermined level because the zener diode D, is connected in parallel with the capacitor C. Therefore whether the voltage of the power source E is E, or E in FIG. 2, the voltage charged across the capacitor C is always E The voltage at the junction 0 at the base of the transistor Tr,, the resistor R, and the capacitor C, is higher than the voltage at the junction b at the zener diode D the emitter of the transistor Tr, and the resistor R The voltage at the junction b is lower than the voltage of the power source E by a voltage drop across the zener diode D As a result the base voltage of the NPN transistor Tr is higher than the emitter voltage so that the transistors Tr, Tr and Tr are turned on. The lamp L is thus turned on. Then the discharge of the capacitor C through the resistor R is started so that when the voltage at the junction a and hence the base potential of the transistor Tr becomes lower than the voltage at the junction b and hence the emitter potential, the transistor Tr, is turned off Tr Tr and, thereby turning off the lamp L.

When the voltage of the power source E drops below its rated voltage, the voltage charged across the capacitor C remains unchanged while the voltage at the junction 12 drops below the voltage obtained when the voltage of the power source does not drop. Therefore, the on time of the transistor Tr, becomes longer so that the exposure or on time of the lamp L is also increased. This will be described with reference to FIG. 2 illustrating the charging-discharging characteristic curve of the capacitor C. When the trigger switch SW is closed, the voltage across the capacitor C increases from zero to E, (which is constant because of the zener diode D,). When the lamp L is turned on, the voltage across the capacitor C gradually drops. (The switch SW remains off). When the voltage of the power source E is E the voltage at the junction 12 is E, and the lamp L is turned on for a time interval T When the voltage of the power source E drops to E the voltage at the junction b becomes E and the lamp L is turned on for a time interval T which is longer than the time interval T Second Embodiment, FlG. 3

The second embodiment whose circuit diagram is shown in FIG. 3 is substantially similar in arrangement to the first embodiment except that a transistor Tr with a differentiating circuit consisting of a capacitor C and a resistor R is inserted in place of the switch SW. Therefore when the trigger switch SW is closed, the differentiated voltage is applied to the base of the transistor Tr, so that even when the trigger switch SW is closed for a relatively long time, the lamp L is turned on for a time interval corresponding to the voltage of the power source.

In the first and second embodiments of the present invention, the decrease in quantity of light emitted from the lamp L due to the voltage drop of the power source may be compensated by varying the resistance of the variable resistor R and by selecting suitable zener diodes D and D When the device of the present invention is incorporated in a camera capable of superimposing the data, the service life may be increased and the failure in data superimposition may be prevented.

It is to be understood that instead of the NPN transistors Tr and Tn, and the PNP transistors Trand Tr;;, the PNP and NPN transistors may be used with the polarities of the power source and zener diodes reversed.

What is claimed is: v

1. A Device for controlling a lighting time interval of a lamp, comprising;

a. a DC power source having a nonconstant voltage output;

b. a charging-discharging circuit and a trigger switch connected in series, the series combination being connected in parallel with said DC power source and charged to a constant predetermined voltage when the trigger switch is closed, and then dis charged when the trigger switch is opened; c. a switching circuit having two states connected to said charging-discharging circuit;

d. a lamp connected to said switching circuit so as to be turned on and off in response to the state of said switching circuit;

e. a series circuit comprising a resistor and means for obtaining a constant voltage. said series circuit being connected in parallel with said DC power source to obtain a voltage at the junction of the resistor and the constant voltage means which is the voltage of the DC power source less the constant voltage drop of said constant voltage means; and

f. a switching element having a control terminal and a reference terminal in said switching circuit, said control terminal being connected to said chargingdischarging circuit and said reference terminal being connected to the junction between said means for obtaining a constant voltage and said resistor, the difference between the voltage at the control terminal and the voltage at the reference terminal being an inverse function of said DC power source voltage output and the voltage difference controlling the state of said switching circuit such that said switching circuit turns on said lamp for a longer time interval as said DC power source voltage output decreases.

2. A device as defined in claim 1 wherein said charging-discharging circuit comprises:

a capacitor;

a variable resistor; and

a second means for obtaining a constant voltage, said capacitor, said resistor and said means for obtaining a constant voltage being connected in parallel with each other.

3. A device as defined in claim 1 wherein said trigger switch comprises a switch and circuit means for providing a differentiated output pulse, the duration of said pulse being substantially independent of the duration of activation of said trigger switch.

4. A device as defined in claim 3 wherein said circuit means comprises:

a transistor; and

a differentiating circuit activated by said switch and connected to the base of said transistor.

5. A device as in claim 2 wherein said means for obtaining a constant voltage is a zener diode. 

1. A Device for controlling a lighting time interval of a lamp, comprising; a. a DC power source having a nonconstant voltage output; b. a charging-discharging circuit and a trigger switch connected in series, the series combination being connected in parallel with said DC power source and charged to a constant predetermined voltage when the trigger switch is closed, and then discharged when the trigger switch is opened; c. a switching circuit having two states connected to said charging-discharging circuit; d. a lamp connected to said switching circuit so as to be turned on and off in response to the state of said switching circuit; e. a series circuit comprising a resistor and means for obtaining a constant voltage, said series circuit being connected in parallel with said DC power source to obtain a voltage at the junction of the resistor and the constant voltage means which is the voltage of the DC power source less the constant voltage drop of said constant voltage means; and f. a switching element having a control terminal and a reference terminal in said switching circuit, said control terminal being connected to said charging-discharging circuit and said reference terminal being connected to the junction between said means for obtaining a constant voltage and said resistor, the difference between the voltage at the control terminal and the voltage at the reference terminal being an inverse function of said DC power source voltage output and the voltage difference controlling the state of said switching circuit such that said switching circuit turns on said lamp for a longer time interval as said DC power source voltage output decreases.
 2. A device as defined in claim 1 wherein said charging-discharging circuit comprises: a capacitor; a variable resistor; and a second means for obtaining a constant voltage, said capacitor, said resistor and said means for obtaining a constant voltage being connected in parallel with each other.
 3. A device as defined in claim 1 wherein said trigger switch comprises a switch and circuit means for providing a differentiated output pulse, the duration of said pulse being substantially independent of the duration of activation of said trigger switch.
 4. A device as defined in claim 3 wherein said circuit means comprises: a transistor; and a differentiating circuit activated by said switch and connected to the base of said transistor.
 5. A device as in claim 2 wherein said means for obtaining a constant voltage is a zener diode. 